I have started using this in my classes for the common reading at my college this year: Huckleberry Finn. This is the most complete site on Twain I've seen. I found especially riveting the section on the various depictions of Jim over the years and what those portraits say about the culture of the times in which they were created.
I create links to certain aspects of the site on my on-line schedule, which directs the students to certain areas I want them to see/use/experience. For example, the Mark Twain "quiz" was a fun way for them to introduce themselves to aspects of Twain's life before we even started reading.
This has an easy point-and-click style. Nearly everyone, especially computer-saavy students, are able to use this site. The main difficulty (we should always have this problem) is the overwhelming amount of material that the site covers.